<?xml version="1.0" encoding="utf-8"?>
 <grammar xmlns="http://relaxng.org/ns/structure/1.0">
   <start>
     <group>
       <attribute name="id">
         <parentRef name="id-content"/>
       </attribute>
       <attribute name="available">
         <parentRef name="available-content"/>
       </attribute>
       <element name="isbn">
         <parentRef name="isbn-content"/>
       </element>
       <element name="title">
         <parentRef name="title-content"/>
       </element>
       <zeroOrMore>
         <element name="author">
           <parentRef name="author-content"/>
         </element>
       </zeroOrMore>
       <zeroOrMore>
         <element name="character">
           <parentRef name="character-content"/>
         </element>
       </zeroOrMore>
     </group>
   </start>
 </grammar>
